To evaluate methionine requirement when cystine is provided by diet. Methionine balance (at requirement intake) will be compared with lower methionine intake plus cystine. By administering orally multi-labeled isotope of methionine and cysteine, the investigators will be able to obtain information on methionine-cystine cycle and further investigate the cystine sparing role in methionine utilization. During an additional tracer study, tracers will be infused, I.V., to provide information related to the obligatory oxidative.